Therapeutic cloning and stem cell therapy
Cell biology • Cell division
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Definition and types of stem cells
Embryonic stem cells originate in very early embryos and can develop into most specialised cell types of the body; they are described as totipotent or pluripotent depending on stage. Adult stem cells occur in specific tissues and are multipotent, so they only form cell types of their tissue of origin. A clone is an organism or cell population with identical genes to its source; artificial cloning methods produce genetically identical cells or organisms .
Overview of therapeutic cloning (main idea)
Therapeutic cloning produces an embryo that is genetically identical to a patient. The process uses the patient’s DNA to create an embryo in vitro and then harvests embryonic stem cells from that embryo. The resulting stem cells share the patient’s genes, making them a genetic match for later medical use.
Key steps in therapeutic cloning (somatic cell nuclear transfer)
1. A somatic cell is taken from the patient and the nucleus, which contains the patient’s DNA, is isolated. 2. An egg cell from a donor is enucleated (its nucleus removed). 3. The patient nucleus is inserted into the enucleated egg. 4. The reconstructed egg receives a stimulus that triggers cell division and develops into a blastocyst-stage embryo. 5. Embryonic stem cells are removed from the blastocyst and grown in sterile culture. These cultured cells are genetically identical to the patient and can be expanded by mitosis for research or therapy.
How cloned embryonic stem cells become specialised cells
Cultured embryonic stem cells remain undifferentiated when maintained with supporting growth medium and signals. Specific combinations of growth factors, signalling molecules and substrate conditions cause directed differentiation: cause → effect. For example, particular growth factors cause stem cells to switch on gene programs for nerve cells; different signals cause them to become insulin-producing pancreatic cells. Controlled culture conditions and stepwise exposure to signals guide stem cells through developmental pathways to the required specialised cell type.
Why therapeutic-embryo stem cells avoid rejection
Immune rejection occurs when the recipient’s immune system recognises transplanted cells as foreign, commonly because of different surface proteins (major histocompatibility complex, MHC). Stem cells derived from a therapeutic embryo contain the same genes as the patient, so their cell-surface proteins match the patient’s MHC profile. Matching MHC proteins reduces immune detection and therefore lowers the chance of rejection after transplantation.
Practical uses and examples
Patient-matched embryonic stem cells offer potential treatments for conditions that require cell replacement, such as damaged spinal cord tissue, defective pancreatic islet cells in diabetes, or certain degenerative eye conditions. Cultured stem cells also provide platforms for drug testing and disease modelling since they reproduce patient-specific genetic backgrounds .
Risks, limitations and ethical factors
Directed differentiation can be difficult to control; incomplete differentiation or genetic abnormalities can cause tumour formation after transplantation. Sterile culture conditions are essential to avoid infection. Ethical concerns arise from the use of human embryos for stem cell extraction. Alternative approaches, such as induced pluripotent stem cells (iPSCs) produced by reprogramming adult cells, reduce some ethical concerns but introduce other technical limitations.
Key notes
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